Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion
Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion
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DOI：
10.1093/petrology/egu035
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发表时间：
2014-08-01
影响因子：
3.9
通讯作者：
Humphreys, Madeleine C. S.
中科院分区：
文献类型：
--
作者：
Bufe, N. Aaron;Holness, Marian B.;Humphreys, Madeleine C. S.
东格陵兰岛第三纪skaerggaard侵入于前寒武纪角闪岩相片岩与上覆第三纪高原玄武岩之间的浅层地壳不整合处。石英长石片麻岩的最高接触变质温度是利用显微结构观察（光学和阴极发光）和石英中钛（TitaniQ）温度计在侵入体西部接触的光圈上进行的两次样品穿越中确定的。片麻岩中石英的再结晶发生在距离接触点390 ~ 340 m之间，而在距离接触点130 m以下的片麻岩中则发生了h2o助熔剂熔化（此处温度与675A℃相似）。石英在接触处记录的最高温度类似于865 +/- 70℃，尽管熔体在所有尺度上均不均匀分布，但在某些样品中熔体分数达到50-60 vol. %。角闪岩相基性片麻岩的小带在接触面附近与含辉石岩的无水角砾岩广泛反应，而超过130 m的角砾岩则被绿片岩相组合覆盖。角闪岩-绿片岩相反应的预期温度与相邻石英长石片麻岩温度的差异与直接从基性片麻岩中形成无水辉石角砾岩相一致，低品位绿片岩相组合是在有限热液活动建立后逆行形成的。片麻岩中的脱挥发反应不太可能产生足够的水来解释在侵入岩的边缘边界系列中形成的伟晶岩特征。一个简单的一维热模型，忽略任何热液循环的热平流，拟合了通过光晕的最高温度剖面。片麻岩的温度普遍低于以前报道的侵入岩壁上接触变质玄武岩的温度，这与结晶潜热的非均匀释放是一致的。
The Tertiary Skaergaard intrusion, East Greenland, intruded at the shallow crustal unconformity between Precambrian amphibolite-facies gneisses and overlying Tertiary Plateau Basalts. Maximum contact metamorphic temperatures in quartzo-feldspathic gneisses were determined in two sample traverses across the aureole on the western contact of the intrusion using a combination of microstructural observations (both optical and cathodoluminescence) and the titanium-in-quartz (TitaniQ) thermometer. The onset of recrystallization of the quartz in the gneisses occurred between 390 and 340 m from the contact whereas H2O-fluxed melting occurred in gneisses closer than 130 m from the contact (where T > similar to 675A degrees C). The maximum temperature recorded by quartz at the contact is similar to 865 +/- 70A degrees C. Melt fractions reach 50-60 vol. % in some samples although the melt is heterogeneously distributed on all scales. Minor bands of amphibolite-facies mafic gneiss are extensively reacted to an anhydrous pyroxene-bearing hornfels close to the contact, whereas those further than similar to 130 m are overprinted by a greenschist-facies assemblage. Discrepancies between the expected temperature for the amphibolite- to greenschist-facies reaction and temperatures obtained from adjacent quartzo-feldspathic gneisses are consistent with the formation of the anhydrous pyroxene hornfels directly from the mafic gneiss, with the lower-grade greenschist-facies assemblage forming on the retrograde path after the establishment of limited hydrothermal activity. It is unlikely that devolatilization reactions in the gneiss produced sufficient H2O to account for the pegmatitic features formed in the Marginal Border Series in the intrusion. A simple one-dimensional thermal model, neglecting any advection of heat by hydrothermal circulation, was fitted to the profile of maximum temperature through the aureole. The generally lower temperatures seen in the gneiss compared with those previously reported for the contact metamorphosed basalts higher up the walls of the intrusion are consistent with a heterogeneous release of latent heat of crystallization.